A nitride compound semiconductor represented by a GaN compound semiconductor is known as a wide-bandgap semiconductor wherein the bandgap of GaN is as large as 3.4 eV at room temperature. Since such a semiconductor has higher saturation velocity and breakdown electric field than that of the other semiconductors, such as GaAs, and Si semiconductors. The nitride compound semiconductor is expected for high-frequency and high-power electronic devices. It is one of features of a GaN based semiconductor that internal field is formed by spontaneous polarization and piezo polarization in a direction perpendicular to the formed (0001) face, and in an AlGaN/GaN hetero structure, a sheet carrier concentration of at least 1×1013 cm−2 is obtained in an undoped state. A hetero junction field effect transistor (HFET) utilizing these features has been fabricated using two-dimensional electron gas in the hetero interface, and the excellent high-frequency operation thereof has been reported. As described above, GaN based transistors heretofore reported are mainly planar field effect transistors. To realize further high-speed operation, a vertical bipolar transistor that can shorten carrier transit time is more promising than a planar transistor. In existing GaN planar field effect transistors, negative bias must be supplied to the gate electrode and positive bias must be supplied to the drain electrode, for which both positive and negative voltage power sources are required. Therefore, the downsizing of, for example, radio communication systems is difficult; however, since a bipolar transistor requires only a positive voltage power source, the downsizing of the systems becomes feasible. Heretofore, there has been reported that high-gain and high-frequency characteristics of a conventional compound semiconductor represented by GaAs can be improved by a hetero junction bipolar transistor (HBT) wherein the hereto junction is formed between the base and the emitter. In also GaN based semiconductors, it is considered that further high-speed transistors can be realized by forming HBTs in the same manner as in conventional compound semiconductors.
An example of heretofore reported GaN based HBT structures will be described below. An AlN buffer layer, an n-type GaN sub-collector layer, an n-type GaN collector layer, an n-type InGaN layer having an In component gradient layer, an Mg-doped p-type InGaN base layer, and an n-type GaN emitter layer are formed in this order on an SiC substrate. A step is formed in the n-type emitter layer, an Al/Au electrode to be an emitter electrode is formed thereon, a Pd/Au electrode to be a base electrode is formed on the exposed p-type InGaN base layer, and an Al/Au electrode to be a collector electrode is formed so as to contact the n-type GaN sub-collector layer. The feature of the GaN based HBT structure is that the activation of the dopant Mg is improved, and the base resistance which is an important factor to determine high-frequency characteristics is lowered by the InGaN base layer, in place of GaN. In the present situation, a current amplification of 20, which is one of the largest in GaN based HBT, can be realized.